1. Field of the Invention
This invention relates to methods and apparatus for stimulating wells by the injection of fluid therein.
2. Description of the Prior Art
Hydrocarbon wells can be stimulated to increase the flow rate of gas or oil by forming fractures in the pay zone where the oil and gas is located. If numerous long fractures can be formed that radiate in all directions from the well bore, then oil or gas can flow into the fractures and to the well bore. One method which has been used for oil well stimulation is the detonation of an explosive charge in the well. However, this tends to crush a limited region near the well bore rather than to open extended fractures out into the producing zone. Furthermore, the pressure impulse from such an explosion generally cannot be maintained for more than a few milliseconds. Since the maximum velocity of crack propagation in typical rocks is on the order of 5000 feet per second, a crack generally cannot propagate more than a few feet when driven by reasonable amounts of high explosive. Because of the fact that detonation used heretofore caused rock crushing rather than cracks, propellants or slow burning explosives are used for the specified purpose. These are normally enclosed in a container which is placed underneath the fracturing fluid surface where they are ignited. However, because of the slow rise time of the impulse caused by the deflagration of the propellant, not many cracks occur (normally one or two) and the fluid pressure is quickly drained off or reduced by the crack which can propagate the easiest, preventing further crack formation and propagation.
Another method which has been used for well stimulation is hydraulic fracturing, which involves the pumping of a fracturing fluid down through the well using large powerful pumps at the surface of the earth. The pressure of the fluid can create fractures, while sand or other propping agents in the fracturing fluid can hold them open after pumping is stopped. Hydraulic fracturing often is successful in stimulating oil and gas well production, but the amount of stimulation is often limited and the cost is high.
In a typical hydraulic fracturing operation, a group of pumping rigs are parked near the well and used to pump fracturing fluid down the well bore at what might ordinarily be considered a high pressure and volume rate. For example, the group of rigs might pump at a flow rate of thousands of gallons of fluid per minute and at pressures at the top of the well of thousands of pounds per square inch, for a period such as a half hour, until perhaps a hundred thousand gallons have been pumped in. This fluid flow is intended to pass out of the well bore at the pay zone, and thus to create long fractures. In some cases, a propellant charge, such as a canister filled with a solid propellant, is lowered into the well and ignited at the level of the pay zone soon after surface pumping begins. The propellant burns in a period such as several miliseconds to create a very high pressure pulse to help start the fractures. Fluid pumped from the surface can thereafter pass along the fractures to elongate them.
While conventional hydraulic fracturing equipment can create fractures which can improve production, its effectiveness is limited by the pressure and volume of fluid it can deliver at the bottom of the well for injection into the pay stratum. Part of the limitation is due to the limited capacity of even very expensive pumping rigs to supply fluid at very high pressures and volumes. The pressure and volume are also limited by reason of the resistance to very rapid flow of fluid through perhaps thousands of feet of pipe that is only several inches in diameter, between the surface pumps and the production zone.
The limitation of pressure and volume is significant because producing formations are generally weaker in one direction than in other directions. Thus, cracks tend to propagate along only the weakest direction. Furthermore, because of stress concentrations at the tip of a fracture, it takes less pressure to propagate a fracture than to initiate one. Once a fracture has formed, therefore, it is difficult to build up pressure sufficient to create additional fractures, since the fluid tends to flow into and propagate the initial fractures.
If a limited pressure and flow rate of fracturing fluid is supplied as by pumping or propellant actuated fluids there is a tendency to create only two fractures that radiate in opposite directions from the well bore along the weakest direction of the formation. These fractures drain off the fracturing fluid, so that pressure cannot build up to create and extend fractures in other directions that are not quite as weak. If a high pressure and flow rate of fracturing fluid could be supplied at the pay zone level of the well bore, then the fractures which initially open along the weakest direction could not drain off fluid fast enough to lower the pressure to a level below that required for fracture initiation. The pressure would then be high enough to open and extend fractures in other directions.
Conventional hydraulic fracturing is limited also by the high cost of the equipment and labor. Each pumping rig may cost several hundred thousand dollars, so that a group of ten that might be used during an operation may cost several million dollars. The cost of renting and operating such rigs can limit their use. A method for fracturing that utilized equipment which had a minimum capital cost and which could be operated with moderate manpower would find wider utilization, particularly if it could produce superior fracturing.
Rock fracturing occurs in response to a stress wave having a sufficiently high amplitude but which is still below the amplitude which causes rock crushing. However, if the rise time of the stress wave is not sufficiently rapid, the first crack will relieve the stress. Stress waves having a sufficiently rapid rise time are caused by detonating explosives as opposed to deflagrating explosives or propellants. The trouble is however that detonating explosives of the type which are usually used create a rock crushing force rather than a rock fracturing force. Reducing the amount of explosives to avoid crushing rock and still cause fractures reduces the duration of the stress wave so that it is too short to be effective, since the interval of the application of the stress wave is also important. Also the technique of denotation should be one which provides sufficient pumping horsepower at the bottom of the well bore to maintain a high fluid injection pressure in spite of the loss of fluid into the first cracks to be created in order to extend the cracks which are created by the stress wave.